1. Field of the Invention
The present invention relates to a solenoid-operated valve of the type that a spool of a valve section is operated upon movement in the axial direction of a plunger of an electromagnetic drive section and in particular, to a solenoid-operated valve which is suitable for use inside of an oil pan of an electronic controlled automatic transmission for a motor vehicle.
2. Discussion of the Related Art
Heretofore, as solenoid-operated valves of this type, there has been known one described in the item of “Prior Art” of a Japanese unexamined, published patent application No. 1-242884 (1989-242884). In the known solenoid-operated valve, a first solenoid housing (core) and a second solenoid housing (yoke) are arranged serially in axial alignment, and a plunger is slidably guided in a guide bore formed in the solenoid housings. By magnetizing the solenoid housings with a solenoid, the plunger is axially moved against a spring, so that a spool in a spool or valve housing attached to the first solenoid housing (core) is operated. Since the space, defined between an end surface of the plunger and a cover, in the guide bore formed in the second solenoid housing (yoke) is to vary its volume as the plunger moves, it is in communication with the external of the solenoid-operated valve through a supply/drain passage formed at, e.g., the center of the plunger to pass therethrough. In this technology, the plunger is slidably guided in the guide bore, and this may give rise to brining the plunger into a lock when foreign matter gets in-between the internal surface of the guide bore and the external surface of the plunger.
To obviate this problem, in a known solenoid-operated valve described in the body of the aforementioned Japanese application, a plunger is constituted by a movable member made of a magnetic material and a rod. The movable member with a press-fitting hole at the center thereof is inserted into a guide bore formed in solenoid housings to be movable back and forth, and the rod is press-fit into the press-fitting hole of the movable member to be protruded from the movable member toward a spool. One end of the rod is press-fit into a fitting hole formed on one end of the spool so that the movable member is supported by the spool without being contacted with the guide bore.
However, in the latter mentioned technology, it is unavoidable to increase the space between the external surface of the plunger made of a magnetic material and the internal surface (i.e., the guide bore) of the solenoid housings. This causes the magnetic resistance in the magnetic circuit to increase thereby to weaken the magnetization of the solenoid housings and the movable member. Thus, the magnetic attracting force exerted on the plunger is weakened, whereby the operating characteristic of the solenoid valve becomes unstable, or whereby the solenoid has to be enlarged to obtain the same operating characteristic.
A technology shown in FIG. 3 may be conceived of as one which is able to solve the aforementioned drawback. The solenoid-operated valve shown in FIG. 3 is composed of an electromagnetic drive section 1 and a valve section 5. In the electromagnetic drive section 1, a yoke 2a and a core 2b each made of a magnetic material are arranged serially in axial alignment through an air gap (i.e., non-magnetic portion) 2d, and the axial opposite ends of the yoke 2a and the core 2b are magnetically connected with each other through a cover 2c which covers the yoke 2a and the core 2b thereby to constitute a stator 2. A plunger 3 is slidably guided in a bore 2e which is formed in the stator 2 over the yoke 2a and the core 2b, and an electromagnetic coil 4 is provided between the yoke 2a and the core 2b inside the cover 2c. Further, the valve section 5 is constituted by inserting a spool 7 slidably in a valve hole of a valve sleeve 6 attached to the stator 2. The spool 7 is urged toward the plunger 3 by means of a spring 9a interposed between itself and a plug member 9 secured to a forward end portion of the valve sleeve 6 and is brought into abutting engagement with the plunger 3 at a rod portion 7c thereof protruding from a rear end portion thereof. Thus, in the inoperative state, the rear end of the plunger 3 is kept in abutting contact with an inner bottom surface of the cover 2c through a washer, as depicted at the upper half in FIG. 3. When electric current is applied to the electromagnetic coil 4, on the contrary, the stator 2 is magnetized in proportion to the magnitude of the electric current applied thereto. Thus, the plunger 3 is attracted toward the core 2b against the spring 9a thereby to operate the valve section 5 through the movement of the spool 7, as depicted at the lower half in FIG. 3.
This solenoid-operated valve is designed for use, e.g., with the valve sleeve 6 being inserted fluid-tightly into a fitting hole of a valve body (not shown) which is provided inside an oil pan of an automatic transmission. The opening degrees of a supply port 6a and a drain port 6c of a valve sleeve 6 at both sides of a control port 6b are increased or decreased reversely with each other in proportion to the moving amount of the spool 7 having two large-diameter land portions 7a, whereby the oil output from the control port 6b is controlled in pressure as well as in volume. An annular groove which is formed inside the valve sleeve 6 between one of the large-diameter land portions 7a and a small-diameter land portion 7b is isolated from the external thereby to define a feedback chamber 6d, to which the control pressure in the control port 6b is applied through a cutout 6e formed at a part of the external surface of the valve sleeve 6 and also through a communication hole 6f. 
A rear end fluid chamber (E) is defined between the rear end surface of the plunger 3 and the inner bottom surface of the cover 2c in the bore 2e formed in the yoke 2a. An electromagnetic section fluid chamber (F) is formed between the bore 2e of the core 2b and the forward end surface of the plunger 3. The rear end fluid chamber (E) communicates with the electromagnetic section fluid chamber (F) through communication grooves 3a formed over the entire length of the plunger 3. An intermediate fluid chamber (H) is formed between the core 2b and the valve sleeve 6 and communicates with the electromagnetic section fluid chamber (F) through a clearance (G) between a center hole of the core 2b and a rod portion 7c of the spool 7. Further, the intermediate fluid chamber (H) further communicates with the external of the solenoid-operated valve through a communication passage 8. That is, the communication grooves 3a, the electromagnetic section fluid chamber (F), the clearance (G), the intermediate fluid chamber (H) and the communication passage 8 constitute a supply/drain passage, through which the rear end fluid chamber (E) is in fluid communication with the external of the solenoid-operated valve. The communication passage 8 takes the form of a labyrinth which is composed of an annular groove 8a formed coaxially with the intermediate fluid chamber (H) and cutouts 8b, 8c which make the annular groove 8a communicate with the intermediate fluid chamber (H) as well as with the external of the solenoid-operated valve. The rear end fluid chamber (E) varies its volume in proportion to the movement of the plunger 3, and in the case where the solenoid-operated valve is provided inside the oil pan, such variation in volume causes the oil around the valve sleeve 6 to be charged into the rear end fluid chamber (E) through the supply/drain passage or to be discharged therefrom.
In the technology shown in FIG. 3, since the plunger 3 made of a magnetic material is received slidably in the bore 2e formed in the yoke 2a and the core 2b and since the clearance therebetween is small, it does not take place that the magnetic resistance in the magnetic circuit constituted by these members increases to weaken the magnetization of the stator 2 and the plunger 3, and thus it is no longer required to make the magnetic coil 4 large for the same operating characteristic. Further, the surrounding oil around the solenoid-operated valve is charged into or discharged from the electromagnetic section fluid chamber (F) through the communication passage 8, the intermediate fluid chamber (H) and the clearance (G). However, since the communication passage 8 of a labyrinth shape composed of the annular groove 8a and the cutouts 8b, 8c is long and extends to move up and down in the case that the solenoid operated valve is used with the axis thereof extending horizontally or being inclined slightly with respect to a horizontal axis, the foreign matter such as iron powder which floats in the surrounding oil subsides or deposits to be separated from the oil while being moved back and forth through the annular groove 8. In addition to this, since the intermediate fluid chamber (H) has a substantial volume, it does not occur that the foreign matter easily comes into the electromagnetic section fluid chamber (F). It is to be noted that the technology shown in FIG. 3 is presented here for the purpose of comparison and does not constitute any known art. Of course, there cannot be found any literature which shows and describes the construction shown in FIG. 3.
However, in the technology shown in FIG. 3, the oil which passes through the clearance (G) between the center hole of the core 2b and the rod portion 7c of the spool 7 flows toward the left when the plunger 3 is moved toward the right against the spring 9a and toward the right when the plunger 3 is moved toward the left. That is, the moving direction of the rod portion 7c and the flowing direction of the oil passing through the clearance (G) are opposite to each other. Since the oil flow impedes the movement of the spool 7 due to the viscosity resistance, there is raised a problem that the responsivety of the solenoid-operated valve is degraded. This problem emerges remarkably when the viscosity of the oil is large at a low temperature.
In addition, the feedback chamber 6d to which the control pressure from the control port 6b is applied and the intermediate fluid chamber (H) whose inside pressure is zero by being in communication with the external through the communication passage 8 are separated by a fitting portion between the valve hole of the valve sleeve 6 and the small-diameter portion 7b of the spool 7, and the fitting portion has a small clearance for allowing the sliding movement of the spool 7. Thus, minute foreign matter floating in the oil supplied from the supply port 6a is led to the feedback chamber 6d through the cutout 6e and the communication hole 6f and then, leaks from the feedback chamber 6d through the small clearance into the intermediate fluid chamber (H) thereby to increase the amount of the foreign matter in the intermediate fluid chamber (H). Therefore, it may be liable that at an earlier stage than as expected, the foreign matter gets into the clearance between the internal surface of the hole 2e and the external surface of the plunger 3 to bring the plunger 3 into a lock.